The IEEE 802.11e standard provides enhancements to the baseline IEEE 802.11 standard for the support of an admission control function at the quality of service (QoS) access point (QAP). These enhancements distinguish QoS enhanced stations (QSTAs) from non-QoS STAs (STAs), and also distinguish QAPs from non-QoS access points (APs).
The QAP is responsible for implementing scheduling and admission control but is not specified explicitly in the standard. The standard provides the mechanisms by which a QSTA can request admission of a traffic stream (TS) and the QAP can provide notice of the admission or rejection of the request.
The traffic specification (TSPEC) is the primary mechanism for communication of QoS parameters which provides the management link between higher layer QoS protocols and the IEEE 802.11e channel access functions. The TSPEC describes characteristics of traffic streams, such as data rate, packet size, delay, and service interval. TSPEC negotiation between peer medium access control (MAC) layers provides the mechanism for controlling admission, establishment, adjustment and removal of traffic streams.
QSTAs send TSPEC requests to the QAP in the form of add traffic stream (ADDTS) management action frames. The QSTA must request TSPEC for both upstream, (from QSTA to QAP), and downstream, (from QAP to QSTA), flows. The QAP evaluates if there are available resources to meet the requested TSPEC. The QAP can respond by offering the QSTA an alternate TSPEC, (perhaps with lower performance QoS parameters), or the QAP may deny the TSPEC request entirely.
Once a TSPEC has been established, the TSPEC may be used for data transfer, and the QAP will meet the TSPECs QoS parameters to the greatest extent possible. A TSPEC may be deleted by the QSTA or the QAP. The QAP may unilaterally delete a QSTA's TSPEC if there are changes in the channel condition reducing available bandwidth, or if higher-priority TSPECs are requesting admittance.
TSPECs are generally created and destroyed based on requests from higher-layer management entities. TSPECs are deleted when the application using the QoS service has completed. Finally, a TSPEC will time out if corresponding traffic does not take place within the timeout defined during the setup
Traffic stream admission control is especially important since there is limited bandwidth available in the wireless medium. Bandwidth access must be controlled to avoid traffic congestion, which can lead to breaking established QoS and drastic degradation of overall throughput.